It's no secret that Congressman Kirk has stumbled in his delicate balancing act between portraying himself as a "moderate" to voters at large while still trying to rally a conservative Republican base.

As Tom Jensen of Public Policy Polling recently explained, Kirk has a math problem:

[Y]ou can't win as a Republican in Illinois without winning over a fair number of Democratic voters and Kirk just isn't doing that right now. He's getting only 5%.

Kirk frequently likens himself to Senator Scott Brown -- indeed, Senator Brown campaigned with Kirk this week and "sees similarities" between himself and Kirk. Both are moderates in blue states. But when it comes to their appeal, there's not much comparison to be made.

To put Kirk's "Brown deficit" into perspective, according to recent polls, only 5% of Democrats support Kirk, while Brown won his Senate race on the backs of some 20% of Democrats who crossed over to vote for the Republican. Moreover, Brown enjoyed a double digit lead with moderates on Election Day, while Kirk is trailing Alexi among that critical voting bloc by double digits.

But Kirk's problem isn't just that, as Jensen put it, he isn't "winning over a fair number of Democratic voters," nor is it just that he's trailing among moderates. Rounding out the trifecta of Kirk's troubles is the entrance of Libertarian Michael Labno into the race. While the presence of a Green Party candidate has already been factored into the race (vote shares for the Green candidate have dropped from 14% in a June PPP poll to 9% in the most recent PPP survey), not a single public poll has gauged the effect of a conservative alternative on the ticket. If recent rumblings from the Republican base are any indication, in a race this tight, a Libertarian could draw serious votes away from Kirk. In fact, just yesterday, ILGOP Chairman Pat Brady expressed concern about that very scenario:

Candidates from the Libertarian Party are expected to appear on the ballot in Illinois this November. [...]

The nominating petitions for the Libertarian and Constitution parties both faced objections, and were both reviewed by election officials. In the end, the Libertarians were found to have collected enough signatures to get on the ballot...The Illinois Republican Party acknowledges it is paying the attorney who led the challenges, though its chairman, Pat Brady, denies the GOP is targeting conservative candidates out of fear they'd steal away votes from his party.

BRADY: Anybody on the ballot poses a threat to the Republican ticket. [...]